1. FIELD OF THE INVENTION
The present invention relates to a sputtering cathode structure for sputtering apparatuses of the planar magnetron type which can increase the life of a planar target plate for sputtering a film material and can control the thickness distribution of a film deposited on a substrate. Further, the present invention relates to a method of controlling magnetic flux generated in the target structure, and to a method of forming films by the use of the target structure.
2. DESCRIPTION OF THE PRIOR ART
In the sputtering technique, a low-pressure gas, e.g. argon, in a space is ionized by glow discharge to form a plasma, and ions in the plasma are accelerated by an electric field generated by a voltage applied between a cathode and an anode to bombard a planar target plate placed on the cathode. Atoms or particles sputtered from the target plate by the bombardment of ions are successively deposited on a substrate place in the vicinity of the anode to form a thin film of a target material.
In this case, in order to assure good qualities for the film deposited on the substrate, to improve the rate of deposition, and to lower the damage to the substrate by electrons, it is important to confine ions and electrons generated by glow discharge in a limited space to a high density, and to effectively transfer the ions in the limited space to the planar target plate.
Therefore, various magnetic field configurations have been studied which can confine the ions in a limited space above the planar target plate to obtain high ion and electron densities.
In recent years, a planar magnetron sputtering apparatus has attained nearly equal deposition rate to that of a conventional resistance heating type vacuum evaporation apparatus, and therefore is widely used in a film forming process for mass production of this films for use in thin film integrated circuits and semi-conductor devices. A recent technical trend of planar magnetron sputtering apparatuses is described in, for example, an article by Waits entitled "Planar magnetron sputtering", J. Vac. Soc. Technol. 15 (2), March/April, 1978, pp. 179 to 187.
FIG. 1 is a cross sectional view for explaining an outline of a structure including a planar target material plate and its peripheral members, namely, a sputtering cathode structure of a well-known conventional planar magnetron sputtering apparatus. In this conventional sputtering apparatus, a ring-shaped magnetic pole portion 2 and a columnar magnet 3 placed at a central part of the magnetic pole ring 2 are magnetically coupled by a yoke 6 on the back side of a planar target material plate 1 (hereinafter referred to as "planar target plate") to form a magnetic circuit. A distribution of lines of magnetic force is established, by the magnetic poles at the pole portions 2 and 3, in a space on the surface side of the planar target plate 1 (in FIG. 1 on the lower side of the plate 1). In more detail, there is established a magnetic field distribution which has such a form as obtained by bisecting a torus by a plane perpendicular to the axis of the torus and by placing the bisecting plane parallel to the surface of the plate 1. In other words, a so-called tunnel-shaped magnetic field distribution 11 is generated. Electrons (not shown) generated by glow discharge are confined in the tunnel-shaped magnetic field distribution 11 so that a high ion density is obtained. The ions are accelerated by an electric field which is approximately perpendicular to the surface of the planar target plate 1 and is generated by a voltage applied between a ring-shaped anode 10 and a disc-like cathode 7, to bombard the surface of the target plate 1. The anode 10 may be directly connected to a directly grounded shield 9 or may be slightly positively biased. As a result, atoms or particles of the target material are successively sputtered from the surface of the target plate 1. Thus, an erosion region 12 is formed in the surface of the plate 1. Incidentally, the cathode body 7 is disposed on the back side of the planar target plate 1 and is electrically connected to a negative high voltage source, and the anode 9 is fixed to the cathode 7 through an insulating spacer 8. Further, reference numeral 5 designates a water cooling mechanism.
As can be seen from the above-mentioned explanation, erosion proceeds with the elapse of time in a sputtering process, and thus the erosion region 12 is formed. In the sputtering cathode structure shown in FIG. 1, the above-mentioned erosion proceeds only in a specified region of the planar target plate, and therefore only a portion of the planar target plate corresponding to the erosion region is used to form a deposited film.
Owing to the erosion region formation mechanism, although a film having a uniform thickness can be obtained in an initial stage, the amount of atoms sputtered from the planar target plate and the direction in which the target material is sputtered, vary with the elapse of time, and therefore the thickness of a film deposited on a substrate (not shown) is not uniform when the erosive action has proceeded. That is, a film which is deposited on the substrate when the erosive action has proceeded, has a thickness distribution having a cross-sectional form of an upwardly convex curve with a central part being downwardly convex, i.e. saddle-like shape as explained later. Accordingly, it is not possible to obtain a deposited film having a desired thickness distribution, e.g. uniform distribution.
In the case where a film having a large thickness is required, or in the case where it is required to perform a long-time sputtering operation which is important from the practical point of view, the planar target plate of the conventional planar magnetron sputtering apparatus cannot be employed due to the above-mentioned localized erosion in the planar target plate and a nonuniform distribution of thickness of a deposited film. That is, the conventional sputtering process is restricted in operating time. In order to eliminate the above-mentioned drawbacks, it has been proposed to vary the magnetic field distribution 11 so that the erosion region 12 is formed in a large surface portion of the planar target plate 1 (refer to U.S. Pat. No. 3,956,093).
The theoretical background or technical thought of this proposal, as is described in column 1, lines 53 to 57 to U.S. Pat. No. 3,956,093, resides in that maximum target erosion is generated at a region substantially aligned with and underlying the point or region over which magnetic flux lines are parallel to the target plate. In more detail, an apparatus is claimed in claim 3 of the above-referred patent which comprises anode means spaced from a planar sputtering source for establishing an electrostatic field therebetween, first magnet means for providing flux lines exiting the source and returning thereto along a curved path thereby defining an erosion region on the source in a closed loop configuration, second magnet means adapted to produce an auxiliary, variable magnetic field in a direction substantially normal to the source in the presence of the flux lines such that upon variation of the variable magnetic field, the location at which resultant flux lines are generally parallel to the source is continuously translated across the erosion region whereby the source is eroded to a generally uniform depth substantially throughout the erosion region. Further, an embodiment in which an electromagnet serving as the second magnet means is arranged separately, is disclosed in the above patent. Besides, a Japanese Patent Application Laid-open Specification (No. 7586/1978) discloses a technique in which magnet means itself is moved mechanically.
The present inventors have restudied the technical thought and technical means disclosed in the patent and laid-open specification with reference to experimental facts which were obtained by the present inventors. As a result thereof, a sputtering cathode structure and a method of controlling magnetic flux generated in the sputtering cathode structure have been obtained which can enhance the effect obtained by the technical means disclosed in the patent and laid-open specification. Further, a technique has been obtained which can freely control the thickness distribution of a deposited thin film.